How to create diatonic chords in G major scale?
G Major Scale
First, let’s look at the G major scale. G – A – B – C – D – E – F♯ – G are the notes of the G major scale.

Diatonic chords are formed by stacking two generic third notes above each scale note.

G Major Diatonic Chords
These are the seven major scale diatonic chords that come from the G major scale.

Each major scale diatonic chord is labelled with a roman numeral number:

All major scales follow this same chord quality pattern:
I, ii, iii, IV, V, vi, viiº
We use uppercase roman numeral numbers to represent major chords, lowercase to represent minor chords, uppercase with a small plus sign to represent augmented chords, and lowercase with a small circle to represent diminished chords.
First, fourth and fifth chords in major scale will always be major chords. Second, third and sixth chords will always be minor chords. Seventh chord will always be diminished chord.
The seven diatonic chords in the G major key are:
I. G – B – D (G major chord)
ii. A – C – E (A minor chord)
iii. B – D – F♯ (B minor chord)
IV. C – E – G (C major chord)
V. D – F♯ – A (D major chord)
vi. E – G – B (E minor chord)
viiº. F♯ – A – C (F Sharp diminished chord)
These are the chords that harmonize the G major scale.
Major Scale Diatonic Chords in All 12 Major Keys
Diatonic Chords of C Major Scale
Diatonic Chords of C Sharp Major Scale / D Flat Major Scale
Diatonic Chords of D Major Scale
Diatonic Chords of E Flat Major Scale
Diatonic Chords of E Major Scale
Diatonic Chords of F Major Scale
Diatonic Chords of F Sharp Major Scale / G Flat Major Scale
Diatonic Chords of G Major Scale
Diatonic Chords of A Flat Major Scale
Diatonic Chords of A Major Scale
Diatonic Chords of B Flat Major Scale
Diatonic Chords of B Major Scale / C Flat Major Scale
Piano Chord Progression Poster
This practical piano chord progression poster from Amazon is the perfect reference chart of all 24 major and minor chord sets — The Really Useful Piano Chord Progression Poster – A1 Size – Folded Version